This application claims priority of PCT application Serial No. PCT/EP01/01662, filed Feb. 15, 2001 and German application No. 100 07 212.7 of Feb. 17, 2000, the complete disclosures of which are hereby incorporated by reference.
a) Field of the Invention
The invention is directed to a method for the acceleration of the adjusting movement in a positioning system in which at least one stepping motor is controlled at a variable control frequency f, the adjusting speed is dependent upon the control frequency f, and a positioning step of the adjusting movement is initiated at every period of the control frequency f, wherein the total number of successive positioning steps corresponds to the length of an adjustment area and a determined position within the adjustment area is assigned to every positioning step.
b) Description of the Related Art
It is known to transform the step angles defined for a stepping motor into increments of a longitudinal movement and, on this basis, to operate positioning systems by means of which displacements of device component groups are carried out with high accuracy along an adjustment area or a path. With a suitable arrangement of the coils and phase control of the stepping motor, a sensitive control can be achieved and, therefore, the rotational movement can be advanced in step angles which are so small that even in optical precision instruments such as zoom devices in microscopes the precise positioning of the zoom groups required for changing the magnification while retaining imaging sharpness is achieved.
In the current state of development, typical step angles are 3.75xc2x0 in permanently excited stepping motors, 1.8xc2x0 in hybrid stepping motors and 1xc2x0 in variable reluctance stepping motors. Generally, spindle systems are used to transform the rotational movement into a longitudinal movement.
A problem which must always be taken into consideration when configuring positioning systems with stepping motors consists in that the available torque decreases as the stepping frequency increases because of the limited current rise rate in the turns of the motor winding. Since part of the torque is used for the acceleration of the external mass, e.g., of a zoom group to be displaced, the control frequency range available for starting and stopping decreases as the moment of inertia increases.
Provided the stepping motor is operated with a control frequency in the start and stop frequency range, it can be stopped at any time without step losses. However, in the acceleration frequency range lying within the upper start limit frequency and operating limit frequency, it is impossible to stop the stepping motor without step loss.
Accordingly, in order to prevent inaccuracies in positioning and to achieve short adjustment times at a given load moment, it is necessary to increase the control frequency successively over the start frequency range and the acceleration frequency range to the operating limit frequency, i.e., acceleration must be carried out step by step until the maximum possible speed is reached when controlling at the operating limit frequency. Conversely, the control frequency must be reduced down to the stop frequency range already before reaching the target position of a component group to be displaced and the stepping motor must accordingly be braked.
The displacement must usually be carried out as quickly as possible and the respective component group must be brought quickly into a given position. In order to accomplish this, acceleration functions and deceleration functions are given in the form of curve paths, also known within technical circles as acceleration and deceleration ramps.
In following the path of an acceleration ramp of this kind, the stepping motors are accelerated by increasing the control frequency up to its maximum rotation speed, wherein the possible increase in the control frequency from step to step is always dependent upon the technical characteristics or efficiency of the respective stepping motor and upon the coupled load.
To this extent, acceleration functions which run linearly are given for ranges of rotation speed in which the step frequency/torque characteristic has an approximately linear function. At higher rotation speed ranges, the acceleration function is based on exponential forms because the latter can be adapted very well to the curve of the step frequency/torque characteristic.
When an adjustment area extending, for example, over a straight path is broken down into individual increments whose quantity is proportional to the step angles and which will be referred to hereinafter as positioning steps, extreme positions can be determined within an adjustment area in which a component group is to he displaced by a definitely predetermined sequence of movements, such extreme positions being starting positions, stopping positions and turning or reversing positions, wherein the movement direction of the component group is reversed in the latter position.
When the component group approaches a stopping or reversing position during displacement, the control frequency must be reduced promptly in the stop frequency range.
Accordingly, it can be indicated for any position within the adjustment area, depending on its distance to one of the extreme positions, whether the control frequency must subsequently be increased, maintained or reduced in the movement direction in order to achieve the operating mode that is optimum with respect to adjustment speed and positioning accuracy.
In the method known from the prior art for controlling stepping motors, the stopping position possible at the time is always compared for this purpose during the adjusting movement to the extreme position to be moved to and the braking process is initiated by reducing the control frequency when the possible stopping position and the extreme position coincide. However, it is disadvantageous that this procedure requires a relatively large storage capacity and calculating capacity.
On the other hand, in another known method, the extreme positions are stored and it is calculated beforehand during the adjusting movement for every positioning step whether or not the current speed lies within the permissible range. Depending on this, it is decided whether the speed should be accelerated, stopped or decelerated. However, the storage capacity and the necessary calculating time can also not be substantially reduced in this way.
The disadvantages mentioned above are of particular concern when the positioning systems in question comprise a plurality of stepping motors for adjusting movements of different component groups. WO 99/60436 describes an xe2x80x9carrangement for direct control of the movement of a zoom system in a stereo microscopexe2x80x9d in which the positioning of the optical zoom groups is provided with separately controlled stepping motors.
In the prior art, with this type of control, the extreme positions of all movement sequences are stored and, as was already mentioned, it is determined in advance for the individual positioning steps whether or not the current speed is in the permissible range which, however, involves the disadvantages already mentioned above.
On this basis, the invention has the object of further developing a method of the type described in the beginning in such a way that the fastest possible movement sequence can be achieved while retaining a high positioning accuracy and reducing technical expenditure.
According to the invention, it is provided in a method of this kind for accelerating the adjusting movements in a positioning system that a permissible control frequency fzul must initially be assigned to every positioning step within an adjustment area. The assigned control frequencies fzul and positioning steps are stored as a data record in a storage in such a way that they can be called up. During the adjusting movement, a comparison is made for each positioning step between the current control frequency fist and the permissible control frequency fzul of the positioning step coming next in the movement direction and, depending on the results of this comparison, the control frequency f is increased for the subsequent step when the result of the comparison is fist less than fzul, the control frequency f is maintained for the subsequent step when the result of the comparison is fist=fzul or the control frequency f is reduced for the subsequent step when the result of the comparison is fist greater than fzul.
By assigning positioning steps and maximum permissible control frequencies fzul, a speed vector is obtained which relates to the entire adjustment area and which contains the permissible speeds of the total system for every path element and every positioning step and which accordingly shows the envelope of the allowed speeds for the adjustment area.
When using the method according to the invention, in order to assess the current speed or to ascertain whether or not the speed can be further increased, it is only necessary to check whether or not the permissible control frequency fzul for the subsequent positioning step is greater than the current control frequency fist. If so, the adjusting movement is accelerated by increasing the control frequency f.
This is also true in an analogous sense for cases where the permissible control frequency fzul assigned to the subsequent positioning step is as great as the current control frequency fist and also for cases in which the permissible control frequency fzul assigned to the subsequent positioning step is less than the current control frequency fzul.
In the former case, the control frequency f remains unchanged; in the latter case the control frequency f is reduced. Since the control frequency f is proportional to the speed of the adjusting movement, as was already mentioned, the speed of the adjusting movement is maintained or reduced.
This results in the advantage that calculating capacity is required only for comparing the current control frequency fist to the control frequency fzul which is permissible for the subsequent positioning step. The calculating expenditure is therefore substantially reduced compared to the prior art. The storage requirement is reduced and, in this way, all adjusting movements are carried out at the maximum possible speed adapted to the respective positioning system.
The adaptation of the highest possible speeds over the entire adjustment area depends on the technical characteristics of the respective stepping motors that are used and, for example, on the mass to be moved. These criteria are taken into account in determining the permissible speeds in relation to every positioning step.
In so doing, it is possible to operate the positioning system in automatic mode as well as by manual control. With manual operation, it is ensured that an arbitrary increase in the adjustment speed can not be carried out because the control frequencies fzul for every path element and for every positioning step are stored in the control circuit and excess increase is blocked.
In a preferred construction of the invention, extreme positions, preferably starting positions, reversing positions and stopping positions, are determined within the adjustment area or within a path and the permissible control frequency fzul for every positioning step is determined depending on its distance from an extreme position.
In this way, it is possible to take into account the ramps during the control of the acceleration and deceleration curve processes. In the area of an acceleration curve, the current control frequency fist is always less than the permissible control frequencies fzul for the next positioning step in the movement direction. In the area of a deceleration curve, on the other hand, the current control frequency fist is always greater than the permissible control frequencies fzul for the next positioning step in the movement direction.
Further, a maximum permissible control frequency fmax which corresponds, for example, to the limit operating frequency of the corresponding stepping motor can be predetermined, so that overdriving of the stepping motor is ruled out. The maximum permissible control frequency fmax is advantageously identical to the permissible control frequency fzul in the portions of a path in which displacing or advancing can be carried out at maximum speed.
In another preferred construction of the invention, different maximum permissible control frequencies fmax and, accordingly, different maximum adjustment speeds are given within the whole adjustment area. For example, it is possible to give a maximum permissible control frequency fgrob for a coarse adjustment at higher speed for a first portion of an adjustment area and a maximum permissible control frequency fein for a fine adjustment at a lower speed for another portion.
The invention will be explained more filly in the following with reference to an embodiment example.